Candidates running for president can easily wreck their campaigns with one serious misstep. Back in 1976, one Democrat said he favored getting rid of criminal penalties for marijuana use. Can you imagine how Americans of that primitive era reacted to his blunder? They elected him.

Once in office, Jimmy Carter didn't abandon his temperate approach to cannabis. He proposed that the federal government stop treating possession of small amounts as a crime, making a sensible but novel argument: "Penalties against possession of a drug should not be more damaging to an individual than the use of the drug itself."

Nothing came of it, of course. Carter's logic was unassailable even 35 years ago, but it has yet to be translated into federal policy. The American experience with prohibition of alcohol proved that we are capable of learning from our mistakes. The experience with prohibition of marijuana proves that we are also capable of doing just the opposite.

The stupidity and futility of the federal war on weed, however, has slowly permeated the mass consciousness. This week, the Gallup organization reported that fully 50 percent of Americans now think marijuana should be made legal. This is the first time since Gallup began asking in 1969 that more Americans support legalization than oppose it.

The shift has shaped drug policy at the state level. Seventeen states have approved medical use of pot, according to the National Organization for the Reform of Marijuana Laws (NORML), and 14 have decriminalized possession of small amounts for personal use—including such staunchly conservative places as Mississippi and Nebraska.

Changes in a permissive direction may bring casual use out of the closet, but they don't elicit the disasters that anti-drug zealots fear. In fact, research indicates that decriminalizing cannabis has only a tiny effect on consumption, if any.

For that matter, hardly anything has an effect. Over the past 30 years, federal spending to fight drugs has risen seven times over, after inflation. Since 1991, arrests for possession of pot have nearly tripled. But all for naught.

As a report last year by the International Centre for Science in Drug Policy noted, more high school students and young adults get high today than 20 years ago. More than 16 million Americans smoke dope at least once a month. Pot is just as available to kids as it ever was, and cheaper than before.

If we had gotten results like this after reducing enforcement, the new policy would be blamed. But politicians who support the drug war never consider that their remedies may be aggravating the disease. They follow the customary formula for government programs: If it works, spend more on it, and if it fails, spend more on it.

During the 2008 campaign, Barack Obama criticized the Bush administration for overriding states on medical marijuana. "What I'm not going to be doing," he vowed, "is using Justice Department resources to try to circumvent state laws on this issue."

For a while it seemed like he meant it. Early on, the Justice Department said it would not waste resources going after sick people who were using cannabis as allowed by states. But recently, federal prosecutors in California have been mobilizing to shut down the state-approved dispensaries that supply those patients.

It's like George W. Bush never left. William Panzer, co-author of the medical marijuana initiative approved by California voters in 1996, told The Los Angeles Times, "The Obama administration has been incredibly disappointing on this issue."

The effort to combat marijuana has served to punish Americans for using a substance that is far less harmful than legal ones. It has enriched organized crime, while fueling endless slaughter by drug cartels in Mexico. It has prevented clinical research on the therapeutic use of cannabis. Its results run the gamut from pathetic ineffectuality to outright harm.

Those facts account for the growing support for legalization, despite ceaseless government propaganda against marijuana. It may seem impossible that cannabis will ever be permitted, regulated and taxed like beer or cigarettes. But when public opinion moves, public policy is bound to follow.

In 1930, the author of the constitutional amendment establishing Prohibition said, "There is as much chance of repealing the Eighteenth Amendment as there is for a hummingbird to fly to the planet Mars with the Washington Monument tied to its tail." Three years later, it was gone.